Melting and casting of transitional metals and alloys

ABSTRACT

A method of melting and casting a transitional metal or an alloy thereof uses an unclad thin-walled graphite crucible having a bottom opening therein in which is positioned a fusible metallic member, usually alike the charge metal in composition, to obturate the opening; charge metal is positioned in the crucible and is rapidly heated by electrical induction to a temperature above its melting temperature without substantially melting the obturating member, and thereby provides a pool of molten metal flowing onto the member to melt a pour hole therethrough through which the molten charge discharges from the crucible.

United States Patent [191 Aldersley et al.

MELTING AND CASTING OF TRANSITIONAL METALS AND ALLOYS Inventors: KennethAldersley, Reading;

Gordon William Telford, Tadley, both of England United Kingdom AtomicEnergy Authority, London, England Filed: May 18, 1973 Appl. No.: 361,490

Assignee:

Foreign Application Priority Data May 23, 1972 Great Britain 2 4184/72U.S. Cl. 75/10 R, 75/65 R Int. Cl C22d 7/04 Field of Search 75/10, 84;164/80, 51

References Cited UNITED STATES PATENTS 8/1966 Sump 75/10 R Dec. 31, 19743,484,840 12/1969 Spoth 164/80 3,504,093 3/1970 Persson 75/10 R3,598,168 8/1971 Clark 164/51 Primary Examiner-L. Dewayne RutledgeAssistant Examiner-Peter D. Rosenberg Attorney, Agent, or Firm-Larson,Taylor and Hinds [5 7] ABSTRACT A method of melting and casting atransitional metal or an alloy thereof uses an unclad thin-walledgraphite crucible having a bottom opening therein in which is positioneda fusible metallic member, usually alike the charge metal incomposition, to obturate the opening; charge metal is positioned in thecrucible and is rapidly heated by electrical induction to a temperatureabove its melting temperature without substantially melting theobturating member, and thereby provides a pool of molten metal flowingonto the member to melt a pour hole therethrough through which themolten charge discharges from the crucible.

10 Claims, 1 Drawing Figure MELTING AND CASTING OF TRANSITIONAL METALSAND ALLOYS BACKGROUND OF THE INVENTION This invention relates to themelting and casting of transitional metals and alloys thereof havinghigh melting temperatures and, in particular, to the melting and castingof titanium and titanium alloys.

US. Pat. No. 3,484,840 discloses a method of melting titanium alloyswhich comprises providing a refractory ceramic crucible having a bottomopening therein, positioning an electrically conductivegraphite sleevewithin said crucible, positioning a titanium alloy disc at the base ofthe sleeve and extending completely across said bottom opening,positioning a charge of the alloy to be melted within said sleeve and inspaced relation to said disc, and rapidly inductively heating suchcharge above its melting point without similarly melting the disc, tothereby provide a pool of molten metal flowing onto the disc and therebymelt an opening through the disc.

The specification explains the disadvantages of skull melting inwater-cooled crucible arc-melting units, namely the need for specializedand high cost equipment, the difficulty in achieving temperature controlwhence the variation in pouring temperatures can cause castingdefects'and the hazard of arc-melting using water-cooled crucibles. Thespecification states that the above-cited method utilises equipmentwhich can be installed and operated by investment casting foundrieswithout substantial capital expenditure, that the method provides a muchbettercontrol of the pouring temperature and that the cost of meltingand pouring titanium, using the method, is significantly reduced.

It has now been unexpectedly established that a container vessel in theform of a refractory ceramic crucible is unnecessary in the melting oftitanium and its alloys and that a graphite crucible thin enough for theinductive power loss therein to be low is capable of holding the moltencharge without bursting, even though not supported within a ceramiccasing.

Accordingly it is an object of the present invention to provide animproved inductive heating method for melting and casting titanium andother transitional metals and the alloys thereof having high meltingtemperatures.

SUMMARY OF THE INVENTION According to the invention a method of meltingand casting a transitional metal or an alloy thereof comprises providingan unclad thin-walled graphite crucible having a bottom opening therein,positioning a fusible metallic member substantially non-deleterious tothe molten charge metal at the bottom of said crucible to obturate saidopening, positioning a charge of said metal or alloy to be melted withinsaid crucible above said member and rapidly heating said charge byelectrical induction to a temperature above its melting temperaturewithout substantially melting said member to thereby provide a pool ofmolten metal flowing onto the member and thereby melt a pour holethrough the member through which the molten charge discharges from thecrucible.

The graphite crucible may have a wall thickness substantially less thanthe electrical reference depth a where a (1/2 7r) 1/ (p/uj) p being theresistivity of graphite (1300 microhm/cm), p. the permeability ofgraphite (l, for a non-ferrous material) and f the frequency of theelectrical supply. In practice, it is found that the wall thickness maybe less than one-fifth the electrical reference depth and preferably isless than one-tenth.

The crucible wall should have a thickness such that the crucible ismechanically strong enough to hold the charge metal yet be such thatenergy generated by the induction coil is absorbed mainly in the metalcharge to be melted, graphite being, electrically, a susceptor material.

The molten charge metal exerts its maximum lateral thrust at a positionclose to the bottom of the crucible: to provide support for the cruciblein this region it is preferred to seat the crucible in a refractorysupporting cup the walls of which form a sliding fit with said crucibleand extend to embrace the crucible over said re gion, said cup having abottom opening enabling molten charge metal to flow from the crucible.Preferably the supporting cup is made of graphite thereby functioning asa heat sink for heat transmitted through the lower wall of the crucible.

To reduce carbon pick-up by the molten metal from the crucible wall thecrucible may be lined with a ceramic material subsequently inert to themolten metal. Suitable materials for use with titanium and titaniumalloys include tungsten, yttria-stabilised zirconia, and yttriaorcalcium-stabilised zirconia mixed with titanium hydride. The materialmay be present as a coating applied by plasma spraying. Alternatively,freestanding liners fabricated from these materials may be inserted intothe crucible to be in contact with and to protect the crucible wall.

The fusible metallic member may be made from said metal or an alloythereof.

Transitional metals reactive to air in the molten state, such as Ti andits alloys, are preferably inductively heated in vacuo.

The invention also provides apparatus for melting and casting atransitional metal or an alloy thereof comprising an unclad thin-walledgraphite crucible having an opening in the base thereof for thedischarge of molten charge metal there-through, and in which a fusiblemetallic member is locatable to obturate the opening, and an electricalinductor heating coildisposed without said crucible and locatable inrelation thereto such that a charge of said metal or alloy, containablewithin the crucible above said obturating member, is within the field ofsaid coil.

The fusible metallic member may be made from metal or an alloy thereof.

The inductor heating coil may include, in addition to a plurality ofcoil turns providing the magnetic flux for melting, at least one reversewound turn so spaced from said plurality of turns to be positionable inrelation to the rim of the crucible to substantiallycurtain metalsplash-over during meltdown.

By the use of electric induction heating rather than heating by anelectric arc the stirring effect in the molten metal produced byinduction heating ensures that, immediately prior to the melting of themetallic member, the whole of the molten charge is at a substantiallyuniform temperature. As a result of this stirring action the whole of amolten charge, immediately prior to casting, can not only be raised to auniform temperature for efficient casting but can also be made todischarge from the crucible at a substantially constant temperature insuccessive melts. A temperature variation of about 25C in thecasting-temperature, in successive melts, has been found possible.

DESCRIPTION OF THE DRAWING mately to scale and has an unclad thin-walledgraphite crucible 1 located within an inductor coil 2. The crucible issupported by a shallow cup 3 provided with support legs 4. The cup 3 hasa bottom opening 12 within which is located a spout 11 supported byshoulders at its uppermost end. The lower part of the crucible is-asliding fit within the walls of the cup: when a charge of metal 6 isrendered molten by high rateinductive heating the molten charge exertsits maximum lateral thrust against the supporting crucible a shortdistance from the bottom of the crucible; at greater distances from thebottom the metal tends to retract inwards from the crucible wall. Thecup 3 provides lateral support for the crucible over this short distancefrom the bottom and being made of graphite also acts as a localised heatsink. To curtain susception in the cup wall three vertical slots are cutthrough the wall (not shown) the slots being cut at intervals of 120:one'slot is extended to cut through the base of the cup and forms aradial slot therein; this has been sufficientto curtain susception inthe base.

The'crucible 1 has an opening 7 in its base 8 for the discharge of themolten charge from the crucible. The opening 7 is obturated by ametallic member 9 which, in this embodiment, is in the form of a steppedplug seated in the opening. The plug 9 is surrounded by an annularinsert 10 made of graphite: the insert is slotted through vertically(slot not shown) to curtain susception and provides a false bottom withthe plug 9 positioned centrally therein. The spout 11 is also made ofgraphite. It will be apparent that the crucible 1 serves toshort-circuit the slots cut through the cup wall 5: to obviate thiseffect thin coatings of alumina are painted or sprayed on the inside ofthe cup wall and/or on the outside of the crucible where it slides intothe cup.

The inductor coil 12 has 14 turns and, spaced above the crucible 2reverse wound turns 13. The reverse wound turns create a magnetic fieldin opposition to that created by the lower 14 turns and serve to curtainmetal splash-over which can otherwise occur when HF power for high rateheating is applied to the coil 2. The reverse wound turns thus allow therequisite power to be applied without causing an unstable melt.

EXAMPLES OF THE PRESENT PROCESS The apparatus of the present embodimenthas been used for melting Ti and Ti alloys. Short bars of metal 2% insdia X 3 ins were inserted into the 2.6 in dia crucible to rest on thefalse bottom 10, as shown.

The electrical measurements for a typical melt were as follows (thecharge metal was 90% Ti 6% Al 4% alloy) Inductance of 14 turn coil 10.85p.H

Capacity Alter- Alter- Field in nator nator KW KVAR Current CircuitVolts Amps (amps) KVA Empty coil 360.0 350 26 -7 1.90 TO Melt 380.0 350I46 50 8 2.15

The wall thickness of the crucible l was within the range 0.0500.070 insand the base thickness also in the range 0.050-0.070 ins. By using acrucible having these dimensions the crucible is electrically andmechanically thinz" a large proportion of the electrical power isabsorbed by the charge. Moreover, a thin crucible can better i)withstand the thermal shock due to the rapid heat rate developed whenpower is applied and ii) promotea high rate of heat loss by radiation'thus effectively cooling the inner crucible surface. If the innercrucible surface can be kept chilled by high radiant heat loss, the rateof reaction between the crucible and a melt can be substantiallyreduced.

Thus the requirements for a crucible as used in the present inventionare i) a capability to hold the charge ii) low power consumption andiii) high radiant heat loss. It will be apparent that the need for acrucible of sufficient strength to hold the charge is the limitingfactor.

In practising the present invention high heating rates and short heattimes are preferably used to reduce carbon pick-up. Carbon pick-up canbe further reduced by lining, or providing liners for, the inner surfaceof the crucible. The crucible can be lined with (for Ti and Ti alloys)tungsten, yttria-stabilised zirconia, or yttriaor calcium-stabilisedzirconia with titanium hydride, and these materials can be applied byplasma spraying: alternatively free-standing liners in these materialscan be produced by plasma spraying and the liners slipped inside thecrucible.

Crucibles for use in the present invention can be made, for example, byi) machining from dense graphite masses, ii) machining from porousgraphite and densifying the inner surface with pyrolitic graphite toimpede metal penetration and confer additional strength, iii) depositionof pyrolitic graphite onto a mandrel and iv) spraying graphite intocarbon fibre or cloth wound ona mandrel and consolidating the innersurface of the crucible so formed with pyrolitic graphite. Linedcrucibles canbe made by deposition of liner material onto a mandrelfollowed by a graphite backing produced by,

for example, spraying or by deposition of pyrolitic graphite.

For apparatus of larger size than that of the embodiment to wallthickness of the crucible may be increased within limits: to retain lowpower absorption with thicker-walled crucibles the frequency of the HFpower supply has to be lowered.

The following Table collates heat times with the carbon content andhardness values for charge metal and cast metal when melted in unlinedand lined crucibles. The charge metal was a Ti alloy Ti 6% Al 4% V).

Carbon Content Hardness. VPN

7a Heat Charge Charge Crucible Structure Time metal cast metal metalcast metal Mins Dense Graphite 2.05 0045 0.230 330 345-380 Graphite(-3071 porisity) lined 2.00 0.045 0089-022 330 345-376 with Pyroliticgraphite Dense graphite Liner: Yttria 2.00 0.045 0040-0048 330 272.363stub. Zirconia Dense Graphite Liner: Yttriu 2.50 0045 0042-0060 330309-350 stab. Zirconia Dense Graphite Liner: ytrria 2.40 0.045 0042-0067330 300-355 stab. Zirconia Ti In melting Ti and Ti alloys the apparatusof the present invention is held in vacuo: U.S. Pat. No. 3,484,840 drawsattention to the problem of contamination in the melting and casting oftitanium and mentions the embrittling effect of hydrogen, oxygen, carbonand nitrogen; accordingly air should be excluded from the meltenvironment so far as is practicable.

Although the above embodiment of the invention is concerned withtitanium and its alloys the invention has application to high-meltingtransitional metals in general. Uranium and uranium alloys can, forexample, with advantage be melted and cast according to the invention.It is believed that, like Ti, the reactive nature of many of thesemetals when molten is curtailed by the high radiant heat loss from thethin-walled crucibles of the invention and that by inducing chill at themetalcrucible interface in this manner the rate of reaction in theregion of this interface is substantially reduced: lined or unlinedcrucibles can be-used according to the reactive habit of individualmetals.

What we claim is:

l. A method of melting and pouring as a molten charge a transitionalmetal or an alloy thereof comprising providing an unclad, thin-walledgraphite crucible having a bottom opening therein, positioning a fusiblemetallic member substantially non-deleterious to the molten charge metalat the bottom of said crucible to obturate said opening, positioning acharge of said metal or alloy to be melted within said crucible abovesaid member and rapidly heating said charge by electrical induction to atemperature above its melting temperature without substantially meltingsaid member to thereby provide a pool of molten metal flowing onto themember and thereby melt a pour hole through the member through which themolten charge discharges from the crucible.

2. A method as claimed in claim 1 wherein the graphite crucible has awall thickness substantially less than the electrical reference depth,a, as herein before defined. V W m V 3. A method as claimed in claim 2wherein the crucible wall thickness is less than one-fifth theelectrical e ttsqsi tt h-w 4. A method as claimed in claim 3 wherein thecrucible wall thickness is less than one-tenth the electrical ef r n slqhi wall ofthe crucible is lined with a ceramic material subt atielly. irttethsmateriali y 8. A method as claimed in claim 7 wherein, in themelting of Ti and Ti alloys, the ceramic material is chosen fromtungsten, yttria-stabilised zirconia and yttriaor calcium-stabilisedzirconia mixed with titau hyd de.

9. A method as claimed in claim 7 wherein the crucible lining isprovided by plasma spraying a coating of' the ceramic material onto theinner wall of the crucible.

10. A method as claimed in claim 7 wherein the crucible lining isprovided by inserting a free-standing liner, fabricated from the ceramicmaterial, into the crucible.

1. A METHOD OF MELTING AND POURING AS A MOLTEN CHARGE A TRANSITIONALMETAL OR AN ALLOY THEREOF COMPRISING PROVIDING AN UNCLAD, THIN-WALLEDGRAPHITE CRUCIBLE HAVING A BOTTOM OPENING THEREIN, POSITIONING A FUSIBLEMETALLIC MEMBER SUBSTANTIALLY NON-DELETERIOUS TO THE MOLTEN CHARGE METALAT THE BOTTOM OF SAID CRUCIBLE TO OBTURATE SAID OPENING, POSITIONING ACHARGE OF SAID METAL OR ALLOY TO BE MELTED WITHIN SAID CRUCIBLE ABOVESAID MEMBER AND RAPIDLY HEATING SAID CHARGE BY ELECTRICAL INDUCTION TO ATEMPERATURE ABOVE ITS MELTING TEOPERATURE WITHOUT SUBSTANTIALLY MELTINGSAID MEMBER TO THEREBY PROVIDE A POOL OF MOLTEN METAL FLOWING ONTO THEMEMBER AND THEREBY MELT A POUR HOLE THROUGH THE MEMBER THROUGH WHICH THEMOLTEN CHARGE DISCHARGES FROM THE CRUCIBLE.
 2. A method as claimed inclaim 1 wherein the graphite crucible has a wall thickness substantiallyless than the electrical reference depth, Alpha , as herein beforedefined.
 3. A method as claimed in claim 2 wherein the crucible wallthickness is less than one-fifth the electrical reference depth.
 4. Amethod as claimed in claim 3 wherein the crucible wall thickness is lessthan one-tenth the electrical reference depth.
 5. A method as claimed inclaim 2 including seating the crucible in a refractory supporting cupthe inner wall of which forms a slidable fit with the crucible outerwall and extends to embrace the lower region of the crucible.
 6. Amethod as claimed in claim 5 in which the supporting cup is a graphitecup.
 7. A method as claimed in claim 5 wherein the inner wall of thecrucible is lined with a ceramic material substantially inert to themolten metal.
 8. A method as claimed in claim 7 wherein, in the meltingof Ti and Ti alloys, the ceramic material is chosen from tungsten,yttria-stabilised zirconia and yttria- or calcium-stabilised zirconiamixed with titanium hydride.
 9. A method as claimed in claim 7 whereinthe crucible lining is provided by plasma spraying a coating of theceramic material onto the inner wall of the crucible.
 10. A method asclaimed in claim 7 wherein the crucible lining is provided by insertinga free-standing liner, fabricated from the ceramic material, into thecrucible.